1. Field of the Invention
The present invention generally relates to magnetic couplings and, in particular, to reducing rotational variations due to self oscillations.
2. Description of the Related Art
Magnetic couplings can be used to transmit rotary motion from one rotatable element to another. A typical magnetic coupling includes two movers. The first mover surrounds a portion of the second mover. The first and second movers both include magnets in the region where they overlap. As is known in the art, the magnets are arranged such that rotation of one of the movers causes the other mover to rotate due to attraction and repulsive forces between the magnets.
One advantage of magnetic couplings is that they can transmit rotary motion from one mover to another without the two movers mechanically contacting each other. This can be useful in situations where a shaft or other mover located in a sealed environment needs to be rotated. An example of such a case can occur in context of drilling a borehole into the earth. In such a case, a bottom hole assembly (BHA) of drill string may require power. The power can be generated by an alternator in the BHA. Given the harsh conditions that exist in a borehole, it is desirable that the alternator be protected from drilling fluid and enclosed in a sealed environment. To this end, a magnetic coupling can be attached to the shaft of the alternator. The magnetic coupling includes an inner rotor having magnets surrounded by an outer housing that also includes magnets. The outer housing can be coupled to the alternator such that the combination forms a sealed environment. The outer housing is fixedly coupled to a turbine. Drilling mud is pumped through the turbine causing it, the outer housing of the alternator to rotate. The magnets in the outer housing and the magnets on the rotor interact such that the rotation of the outer housing causes the rotor to rotate. The rotation can be used to generate electricity for the BHA. As is known in the art, rather than being coupled to an alternator, the magnetic coupling can be attached to any shaft driven devices such as, for example, a pump.